Vehicles such as aircraft, missiles, drones, etc. (hereafter referred to as "aircraft" or "vehicles") are generally provided with a flight control system so that the orientation of the vehicle during flight can be controlled along the three principle axes, namely, yaw, pitch, and roll. The pitch axis, which in an aircraft extends along the wingspan, defines a degree to which the nose of the vehicle is pointed above or below the horizon. The roll axis extends along the length of the aircraft and defines the degree to which the wings of the aircraft are banked. The yaw axis is perpendicular to both the pitch axis and the roll axis.
The control system provides a means for varying the orientation of the aircraft during flight so that, for instance, the wings can be banked to change a direction of flight, or the nose can be raised or lowered to change the altitude. In one type of conventionally configured aircraft, the control system can include: ailerons for controlling the roll angle (bank) of the aircraft in response to a rotation of a control wheel; an elevator for controlling the pitch angle of the aircraft in response to a pushing or pulling on the control yoke; and a rudder for controlling the yaw angle of the aircraft in response to an input to the control pedals.
Other aircraft and other types of vehicles can employ different control devices such as canards, rotatable fins or wings, spoilers, moveable tail surfaces, etc., to affect the orientation of the vehicle.
While aircraft can be controlled via the control system by a human pilot (often referred to as "man-in-the-loop"), aircraft can also be controlled for a portion of a flight, or for an entire flight, by an autopilot system. In the autopilot situation, the autopilot--as opposed to a human pilot--manipulates the control surfaces in order to control the orientation of the vehicle in response to an established or inputted command. Examples of an inputted signal include, but are not limited to: a course guidance signal inputted to an autopilot, so that the autopilot can control the direction of flight so as to follow the heading or course set; an altitude hold/altitude capture input, so that the autopilot can control the altitude and/or the rate of change of the altitude of the vehicle; and a turn rate command, so that the autopilot will initiate a turn at a requested rate or acceleration amount.
Aircraft can also be controlled in a manner which is not purely manual or purely automatic but where a flight director is used. Where a flight director is used, the flight director receives a command signal from the autopilot which would normally be used to control the flight control elements. However, instead of controlling the flight control elements, the command signal controls the position of a flight director indicator on the pilot's attitude indicator or multi function display, which tells the pilot how to manipulate the flight control elements so that the aircraft flies according to a desired flight profile.
In the cases where an autopilot is used, the autopilot must interpret an inputted guidance or altitude command and, based on that inputted command, output a command for manipulating the flight control elements which is suitable for achieving the desired result. For instance, in response to a guidance command which requests a left turn at a 2 g rate, the autopilot must output a command for a particular control surface--or combination of control surfaces--instructing a direction and amount of control surface movement which is appropriate for achieving the requested 2 g left turn.
The autopilot will generally be implemented with some type of feedback which provides information pertaining to the operating status of the vehicle relative to the guidance command, so that the autopilot can determine when the desired result has been achieved or whether additional flight control manipulation is required to achieve or maintain the desired result.
FIG. 1 is a conceptual block diagram of an autopilot which is implemented as a control system with feedback. As shown, a guidance command is inputted to the autopilot controller 10, which interprets the command and outputs a control surface actuation or deflection command .delta.. The control surface actuation or deflection command .delta. is received by a device 12 such as a control surface servo or actuator, which in turn moves the control surface 14. Generally, the control surface 14 or the control surface actuator 12 will be equipped with a device, such as an encoder (not shown) which outputs the current position of the control surface. The output from the encoder is fed back to the autopilot controller 10 so that the autopilot controller 10 can determine the control surface/actuator deflection output .delta. based on the current position of the control surface 14.
The autopilot controller 10 determines the control surface response which is needed to accomplish the inputted command based, in part, on vehicle condition information which is fed back from, for instance, a sensor 16 on the vehicle. A difference, generally referred to as an error signal, between the feedback signal and the guidance command, is evaluated in order to determine the control surface response which is needed to accomplish the inputted command. Because both the actuation of the control surface as well as the response of the vehicle to the control surface actuation occur over a period of time, the error signal is continually monitored as the control surface is moved so that the autopilot controller can determine when the guidance command has been successfiully performed. The autopilot controller determines that a guidance command has been successfully performed when the error signal goes to zero. So long as a non-zero error signal is received, that indicates a situation where the guidance command request has not yet been accomplished and/or where further control surface actuation may be required.
As an example, this general autopilot system can be described in the context of an autopilot for controlling the altitude of a vehicle where information from an altimeter (sensor 16) would be fed back to the autopilot controller 10. Thus, if a guidance command is inputted which requires a change in the altitude, a non-zero error signal will be generated so long as the current altitude is different from the requested altitude, and the autopilot controller 10 will output control surface commands to change the altitude of the vehicle. When the current altitude of the vehicle matches the commanded altitude, i.e., when the error signal becomes zero, the autopilot controller 10 will cease to output control surface commands or will output control surface commands appropriate to maintain the commanded altitude.
While the example above addresses only one aspect of the vehicle (altitude), it should also be appreciated that the autopilot can control pitch, roll, yaw, or other aspects. Additionally, a multi-axis autopilot can be employed where several of the aspects of the vehicle are simultaneously controlled.
Because, for a given command, the autopilot must output flight control commands which are appropriate for the vehicle on which it operates, each autopilot must, to some degree, be designed for, or tailored to, the specific vehicle for which it is intended.
In designing or tailoring the autopilot, not only must the types of control surfaces which are provided on the vehicle (and the aspect of the vehicle orientation which they effect) be comprehended, but also the vehicle response to varying degrees of control surface deflection need to be addressed so that the autopilot can respond to more or less aggressive maneuvers and so that the autopilot can limit the control outputs to those which will not over stress the vehicle. More sophisticated autopilots can also respond to an inputted command in different manners depending on the operating status of the vehicle at the time of the command. Specifically, depending on the airspeed, angle-of-attack, etc., the output from an autopilot can vary for a given input.
Thus, in order to design or tailor an autopilot for a vehicle, information about the dynamic behavior of the vehicle is generally required. One aspect of particular importance is the relationship between the angle of attack of the aircraft wing (or moveable fin or other type of control surface) and the amount of lift created. For a given airspeed, the lift created by an aircraft wing increases with an increasing angle of attack. A typical relationship between the amount of lift created as a function of the angle of attack is illustrated as curve 20 in FIG. 2. As can be seen from FIG. 2, from a zero angle of attack to some finite value, the lift coefficient versus the angle of attack is at least approximately linear (dashed line 22). For angles of attack beyond this finite value, the lift coefficient no longer increases linearly and at some point flattens and then begins to decrease. At the point where the lift coefficient curve reaches its maximum value, the angle of attack is so great that the airflow no longer conforms to the surface of the wing, thereby destroying, or substantially reducing, lift in a condition known as stall. In FIG. 2, the stall point is where the slope of the lift curve declines to zero and is designated the critical angle of attack .alpha..sub.c. As can also be seen, any further increase in the angle of attack beyond the critical angle of attack .alpha..sub.c results in a reduction in the lift coefficient and a negative slope for the portion of the lift curve.
For convenience, FIG. 2 plots lift coefficient C.sub.L which is related to the amount of lift produced by the wing or control surface. Specifically, the lift coefficient C.sub.L for a given aircraft is normally defined as the ratio of the lift force (or weight of the aircraft) divided by the dynamic pressure Q times the wing reference areas. The dynamic pressure, in turn, is a product of the air density and the velocity.
When a wing stalls, not only is lift lost, which can result in the aircraft losing altitude and/or deviating from a desired direction, but also the associated control surface may cease to be effective for controlling the orientation of the vehicle. Thus, in the event of a stall, the altitude of the aircraft may drop and the orientation of the aircraft may be partially or wholly uncontrollable for the duration of the stall.
In some situations, a pilot may be able to reduce the angle of attack of the wing and recover from the stall; however, in other situations, such as where the aircraft is near to the ground, stall recovery cannot be accomplished before the aircraft impacts the ground. Accordingly, for both man-in-the-loop as well as autopilot based operations, there has been a general objective in the prior art of avoiding stall.
Avoidance of stall in practice can be difficult because the precise point at which the wing stalls is not only a function of the angle of attack, but also the airspeed, the load factor, and other variables. Furthermore, the angle of attack is the angle between the chord of the wing (i.e., a line from the leading edge to the trailing edge) and the wing's flight path. Accordingly, the angle of incidence of a wing can be affected by a change in the relative direction of the airflow, such as may occur due to turbulence or wind shear. Accordingly, when a wing is operating near its maximum lift, turbulence or wind shear may cause the critical angle of attack to be inadvertently exceeded with a stall unintentionally resulting. As a result, for both man-in-the-loop and autopilot base systems, stall is avoided not only by not purposefully exceeding the maximum angle of attack but by also maintaining an angle of attack safety margin so that turbulence or wind shear or other factors do not unexpectedly cause a stall.
While the lift versus angle of attack relationship as described above has been known, conventional autopilots have typically been able to control the vehicle only for the portion of the lift versus angle of attack region prior to the point of stall. Additionally, because the relationship between lift and angle of attack is nearly linear for a large portion of the angles of attack approaching the stall point, conventional autopilots often are based on the simplifying assumption that the relationship between lift and angle of attack is linear for all angles of attack.
The conventional autopilot assumption that the relationship between lift and angle of attack is linear for all angles of attack can result in controllability problems if stall should occur and can also result in improper control commands being generated during stall. Specifically, when stall occurs in the conventional autopilot feedback control approach, the force generated by the lifting surface decreases. However, in the case where the autopilot is attempting to command a control surface deflection for generating a greater force than the stalled liftg surface, the error signal will increase after stall and the autopilot will further increase the angle of attack of the lifting surface. While a further increase in the angle of attack would yield an increase in the force generated if the lting surface were not stalled, if the lifting surface is stalled the further increase the angle of attack of the lifting surface actually worsens the stall condition by further reducing the lift generated. Further, as the autopilot inadvertently worsens the stall situation while attempting to increase the force generated, the resulting error signal will continue to grow larger, which suggests to the autopilot that a still greater angle of attack is required.
At least one known autopilot attempts to address the situation of stall by monitoring or estimating the angle of attack of the vehicle and for angles of attack where the coefficient of lift goes to zero or goes negative, a pseudo lift signal is generated and supplied to the autopilot control loop. The pseudo lift signal tracks the actual coefficient of lift characteristics of the vehicle over the angle of attack region where the response is linear. For the region of angle of attack where the lift rate goes to zero or goes negative, the pseudo signal continues to increase linearly. As a result, the autopilot feedback loop does not see the reduced lift signal from the sensor, and thus does not try to further increase the angle of attack in order to achieve the guidance command input. As a result, the control loop is prevented from inadvertently driving the vehicle further into a stall.
While the approach above tends to avoid stall, and prevents the control loop from inadvertently driving the vehicle further into a stall, this approach does not allow the vehicle to be effectively controlled near or at the point of stall.
On the other hand, because the amount of lift generated by the wing increases with increasing angle of attack, the maximum lift of the wing, and hence the maximum performance of the aircraft, is achieved when the wing is very near the point of stall. Operating a wing near the point of maximum lift results in vehicle performance benefits such as shorter takeoff distance, greater load carrying capability, and higher climb rate. Further, operating the control surfaces near the point of maximum lift results in faster, more aggressive turning capability and greater overall maneuverability. As a result, there has been an increasing interest in systems which will allow aircraft and other vehicles to be operated very near to the point of stall. Additionally, because turbulence or wind shear may cause the critical angle of attack to be inadvertently exceeded for a vehicle that is operating very near to the point of stall, there exists a need for an autopilot controller which can operate near the point of stall and also properly control the vehicle at and after the point of stall.